Gas separation membranes are used in various industrial processes including the production of oxygen enriched air, separation of moisture or carbon dioxide from natural gas, and the recovery or capture of a desired gas species from vented gases such as flue gas form coal and natural gas power stations.
The composition of flue gases from power plants varies greatly depending upon the fuel source used, but will typically comprise gas species such as N2, O2, H2O, CO2, SOx, NOx and/or HCl. Gas separation membranes may be used to separate a target gas species from gas mixtures such as that provided by flue gases.
One gas that is a common target gas species to be separated from a mixture of gas species is CO2. In that case, it is often desirable to separate CO2 from gases such as H2, N2, and/or CH4. Other desirable gas separation combinations include O2/N2 (i.e. oxygen gas from nitrogen gas), He/N2 and He/CH4.
Polymers are commonly used for gas separation membranes. For a given polymer to function effectively as a gas separation membrane it needs to meet a number of criteria. One such criterion is an ability for gas to permeate through the polymer membrane so as to achieve a satisfactory gas flux during separation.
A second criterion is for the polymer membrane to provide appropriate selective separation of a target gas species from a mixture of gas species (commonly referred to as the “selectivity” of the membrane). In the simplest case, the selectivity can be defined as the permeability of the target gas (gas A(PA)) over the permeability of the other gas species present (gas B(PB)):                PA/PB         
A third criterion is that the polymer membrane should provide good thermal mechanical properties so as to afford sound structural stability during the separation process, which may be conducted under pressure.
The two criteria of permeability of the membrane to the mixture of gas species, and selectivity of the membrane to the target gas species over other gas species present in the mixture, typically have an inverse relationship. In other words, increasing the permeability of the membrane tends to decrease its selectivity (i.e. as increasing the permeability tends to increase the permeability for all gases). Similarly, increasing the selectivity of the membrane for a target gas species over other gas species present tends to decrease its permeability to the target gas species (i.e. the restriction of flow of non-target gas species through the membrane tends to restrict flow of all gas species, even though the restriction of flow of the target species is not as severe). This phenomenon has been studied, and the upper boundary on the combination of permeability and selectivity has been plotted. The plot of the upper boundary of permeability against selectivity is known as Robeson's upper bound (Journal of Membrane Science, 1991, 62, pg 165).
Considerable research to date has been directed toward developing gas separation membranes that exhibit a suitable balance between permeability and selectivity for viable commercial gas separation processes, coupled with adequate structural stability for use in such processes.
Accordingly, there remains an opportunity to develop new gas separation membranes that exhibit improved properties or offer a practical alternative to known gas separation membranes.